Euglena, the Phototaxic Protozoa

Background

The free living single cell protists Euglena are unique in their ability to obtain energy by mitochondrial respiration (food) or chloroplast photosynthesis (light). Euglena uses their eyespot to orient and direct themselves via flagellar motion toward light sources in a process termed phototaxis. Euglena are microscopic. To the human eye, Euglena swimming in water have the appearance of a greenish haze.

Objective

Students learn to appreciate the diversity of life and consider what constitutes a plant or animal. Students also learn about experimental design by using a control and test group.

Procedure

Wrap one jar completely in opaque paper (taped to fasten). This is the control jar.

Wrap the other jar completely in opaque paper (taped to fasten) except that that you’ve cut a numeral corresponding to the students’ grade level (about 1 inch tall). For example, third graders would cut out the number 3.

Position both jars side-by-side a few inches from the lamp bulb.

Point the lamp at the cut-out portion of the test jar and corresponding covered portion of the control jar. Turn on the lamp.

Wait for 8 to 10 minutes.

Turn off the lamp.

Remove the paper from the control jar.

Remove the paper from the test jar.

Results

No changes are evident in the control jar. In the test jar, Euglena have gathered on the glass next to the cut-out portion of the jar in the shape of the numeral.

Conclusions

Euglena are phototaxic, which means they have directed motility toward a light source. The control jar shows that in the absence of a gradient of light, these organisms are randomly distributed in solution.

During the experiment, teachers can draw a picture of Euglena and review the anatomy of key organelles (mitochondrial, chloroplast, eyespot, flagellum) and discuss their function. You can also discuss experimental design (test versus control conditions) and classification of life: plant, animal, etc. If the classroom has a microscope, students can directly observe the Euglena.

This experiment is a classic. Cutting out the grade number is my variation, which always draws a big cheer when I have performed this experiment with grade school students!